Many devices, such as photodiode detectors, photovoltaic devices, radiation detectors, liquid crystal displays, and the like, include light sensitive materials which convert incident light into electrical energy. Light transmissive electrodes play an important role in such devices. When placed in electrical contact with a light sensitive material, a light transmissive electrode allows incident light to pass through the electrode and strike the light sensitive material. The efficiency of devices, such as photodiode detectors, photovoltaic devices, radiation detectors, liquid crystal displays, and the like, in converting incident light into electrical energy depends in part upon the percentage of light that passes through the light transmissive electrode and reaches the light sensitive material.
Light transmissive electrodes have been formed in a variety of ways. Under one approach, a grid of spaced-apart metal conductors is formed on the surface of the light sensitive material. This approach provides good electrical contact with the light sensitive material. Unfortunately, however, the metal conductors block a significant portion of the incident light. Additionally, because the metal grid covers only a portion of surface of the light sensitive material, use of a metal grid increases the internal resistance to current flow from the light sensitive material to the electrode.
Another approach involves depositing a layer of a light transmissive, electrically conductive oxide onto the light sensitive material. Advantageously, this approach allows a very high percentage of incident light to pass through the electrode and strike the light sensitive material. Moreover, since the light transmissive, electrically conductive oxide may be deposited over the entire surface of the light sensitive material, any internal resistance to current flow from the light sensitive material to the electrode is substantially eliminated.
However, there are drawbacks to using the known light transmissive, electrically conductive oxides as electrodes. For example, deposition of the light transmissive, electrically conductive oxide onto the light sensitive material according to known techniques occurs at high temperatures, i.e., temperatures greater than about 300.degree. C. At such high temperatures, the light transmissive, electrically conductive oxide tends to diffuse into the light sensitive material. Such diffusion can impair or even destroy the ability of the light sensitive material to convert incident light into electrical energy. Moreover, as a result of using such high temperatures, light transmissive, electrically conductive oxides cannot be deposited onto devices comprising flexible polymer supports since the polymer supports tend to undergo chemical and/or physical degradation at such high temperatures.
When deposited at low temperatures, i.e., temperatures below about 300.degree. C., previously known light transmissive, electrically conductive oxide materials show higher resistivity and visible light absorption values relative to light transmissive, electrically conductive oxide materials deposited at higher temperatures. Moreover, when deposited at low temperatures, previously known light transmissive, electrically conductive oxide materials show a sharp increase in resistivity and absorption when exposed to air, especially when exposed to air at temperatures of about 100.degree. C. or more. As a consequence, previously known light transmissive, electrically conductive oxide materials prepared at low temperatures have not been suitable for use as light transmissive electrodes.